The present invention relates to a polycrystal thin film forming method and forming system, more specifically a polycrystal thin film forming method and forming system for forming at low temperature polycrystal thin film on a substrate of low heat resistance temperature.
Furthermore, the present invention relates to a polycrystal silicon thin film forming method, a thin film transistor fabrication method and a liquid crystal display device fabrication method, more specifically a polycrystal silicon thin film forming method which can form polycrystal silicon thin film on a substrate having low heat resistance temperature, a thin film transistor fabrication method for fabricating a thin film transistor using the polycrystal silicon thin film, and a liquid crystal display device for fabricating a liquid crystal display device using the thin film transistor.
Recently, liquid crystal displays (LCDs) using thin film transistors (TFTs) as switch devices for the picture elements because of electric power saving, space saving, high response speed, beautiful display, etc.
Such liquid crystal displays generally use glass substrates, and the thin film transistors are formed on the glass substrate. The channel layers of the thin film transistors are formed of, in many cases, polycrystal silicon thin film.
As a method for forming polycrystal silicon thin film on a glass substrate has been conventionally known a method in which amorphous silicon thin film is formed on a glass substrate and then is subjected to a heat treatment at 600xc2x0 C. for 50 hours to crystallize the amorphous silicon thin film, and polycrystal silicon thin film is formed. In this method nuclei of crystals are grown at the initial stage of the heat treatment, and the nuclei are grown to form polycrystal silicon thin film.
However, in this polycrystal silicon thin film forming method the heat treatment performed at 600xc2x0 C. for about 50 hours deforms the glass substrate. Furthermore, crystal grain of the thus-formed polycrystal silicon thin film have many defects and twins. Thus this method has found it difficult to form high-quality polycrystal silicon thin film having high electron mobility.
It was considered to form polycrystal silicon thin film on a glass substrate at an above 600xc2x0 C. high temperature by CVD (Chemical Vapor Deposition), but the glass substrate was deformed by the high temperature of above 600xc2x0 C., and the thus-formed polycrystal silicon thin film could not have sufficient crystallization.
Then is proposed a method in which amorphous silicon thin film is formed on a glass substrate, and laser beams are applied to the amorphous silicon thin film to form polycrystal silicon thin film. In this method polycrystal silicon thin film is formed in the process of the silicon melted by the laser beams solidifying. The amorphous silicon thin film is melted by the laser beams for a short period of time without heating the glass substrate to a high temperature. Accordingly polycrystal silicon thin film can be formed without deforming the glass substrate.
However, in this proposed polycrystal silicon thin film forming method because silicon solidifies at high speed, polycrystal silicon thin film having large crystal grain diameters cannot be formed. Thin film transistors using the thus-formed polycrystal silicon thin film as the channel layers have electron mobilities so low as about 150 cm2/Vs.
An object of the present invention is to provide a polycrystal silicon thin film forming method which can provide high electron mobility even when the polycrystal silicon thin film is formed at low temperature, a thin film transistor using the polycrystal silicon thin film, and a liquid crystal display device using the thin film transistor.
The above-described object is achieved by a polycrystal thin film forming method comprising the steps of: forming a semiconductor thin film on a substrate; and flowing a heated gas to the semiconductor thin film while applying an energy beam to the semiconductor thin film to melt the semiconductor thin film at a region the gas is being flowed, and crystallizing the semiconductor thin film in its solidification to form a polycrystal thin film. The energy beam is applied while the high-temperature gas is being flowed, whereby the melted semiconductor thin film can have low solidification rate, whereby the polycrystal thin film can have large crystal grain diameters and can have good quality of little defects in crystal grains and little twins.